Eptaxial structure

ABSTRACT

An epitaxial structure includes a substrate having an epitaxial growth surface, a first epitaxial layer, a graphene layer and a second epitaxial layer. The first epitaxial layer is stacked on the epitaxial growth surface. The graphene layer is coated on the first epitaxial layer. The second epitaxial layer is located on the first epitaxial layer and covers the graphene layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application: Application No. 201210122545.0, filed on Apr.25, 2012 in the China Intellectual Property Office, disclosure of whichare incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to an epitaxial structure and a methodfor making the same.

2. Description of Related Art

Light emitting devices such as light emitting diodes (LEDs) use groupIII-V nitride semiconductors, such as gallium nitride (GaN), have beenput into practice.

Since wide GaN substrate cannot be produced, the LEDs have been producedupon a heteroepitaxial substrate such as sapphire. The use of sapphiresubstrate is problematic due to lattice mismatch and thermal expansionmismatch between GaN and the sapphire substrate. One consequence ofthermal expansion mismatch is bowing of the GaN/sapphire substratestructure, which leads in turn to cracking and difficulty in fabricatingdevices with small feature sizes. A method for solving the problem isforming a plurality of grooves on surface of the sapphire substrate bylithography or etching. However, the process of lithography and etchingare complex, high cost, and may pollute the sapphire substrate.

What is needed, therefore, is to provide an epitaxial method for solvingthe problem discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making anepitaxial structure.

FIG. 2 is a schematic view of one embodiment of a graphene layer havinga plurality of circular shaped apertures.

FIG. 3 is a schematic view of one embodiment of a graphene layer havinga plurality of rectangular shaped apertures.

FIG. 4 is a schematic view of one embodiment of a graphene layer havingapertures in different shapes.

FIG. 5 is a schematic view of one embodiment of a plurality of subgraphene layers spaced from each other.

FIG. 6 is a scanning electron microscope (SEM) image of a drawn carbonnanotube film.

FIG. 7 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 6.

FIG. 8 is an SEM image of cross-stacked drawn carbon nanotube films.

FIG. 9 is a process of growing a second epitaxial layer.

FIG. 10 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 1.

FIG. 11 is a schematic, cross-sectional view, along a line XI-XI of FIG.10.

FIG. 12 is an exploded view of an epitaxial structure.

FIG. 13 is a schematic view of the epitaxial structure of FIG. 12.

FIG. 14 is a schematic view of one embodiment of the epitaxialstructure.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present epitaxial structures and methods formaking the same.

Referring to FIG. 1, a method for making an epitaxial structure 10 ofone embodiment includes following steps:

(S11) providing a substrate 100 having an epitaxial growth surface 101;

(S12) epitaxially growing a first epitaxial layer 110 on the epitaxialgrowth surface 101;

(S13) placing a graphene layer 120 on the first epitaxial layer 110; and

(S14) epitaxially growing a second epitaxial layer 130 on the epitaxialgrowth surface 101.

In step (S11), the epitaxial growth surface 101 is used to grow thefirst epitaxial layer 110. The epitaxial growth surface 101 is a cleanand smooth surface. Oxygen and carbon are removed from the surface. Thesubstrate 100 can be a single layer structure or a multiple layerstructure. If the substrate 100 is a single layer structure, thesubstrate 100 can be a single-crystal structure. The single-crystalstructure includes a crystal face which is used as the epitaxial growthsurface 101. The material of the substrate 100 can be SOI (Silicon oninsulator), LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs,InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs,GaAlN, GaAlN, AlInN, GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn or GaP:N.The material of the substrate 100 is not limited, as long as thesubstrate 100 has an epitaxial growth surface 101 on which the firstepitaxial layer 110 can grow. If the substrate 100 is a multiple layerstructure, the substrate 100 should include at least one layer of thesingle-crystal structure mentioned previously. The material of thesubstrate 100 can be selected according to the first epitaxial layer110. In one embodiment, the lattice constant and thermal expansioncoefficient of the substrate 100 is similar to the first epitaxial layer110 thereof in order to improve the quality of the first epitaxial layer110. In another embodiment, the material of the substrate 100 issapphire. The thickness and the shape of the substrate 100 are arbitraryand can be selected according to need.

In step (S12), the first epitaxial layer 110 can be grown by a methodsuch as molecular beam epitaxy, chemical beam epitaxy, reduced pressureepitaxy, low temperature epitaxy, select epitaxy, liquid phasedeposition epitaxy, metal organic vapor phase epitaxy, ultra-high vacuumchemical vapor deposition, hydride vapor phase epitaxy, or metal organicchemical vapor deposition (MOCVD).

The first epitaxial layer 110 is a single crystal layer grown on thepatterned epitaxial growth surface by epitaxy growth method. Thematerial of the first epitaxial layer 110 can be the same as ordifferent from the material of the substrate 100. If the first epitaxiallayer 110 and the substrate 100 are the same material, the firstepitaxial layer 110 is called a homogeneous epitaxial layer. If thefirst epitaxial layer 110 and the substrate 100 have different material,the first epitaxial layer 110 is called a heteroepitaxial epitaxiallayer. The material of the first epitaxial layer 110 can besemiconductor, metal or alloy. The semiconductor can be Si, GaAs, GaN,GaSb, InN, InP, InAs, InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe,GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN,AlGaInP, GaP:Zn, or GaP:N. The metal can be aluminum, platinum, copper,or silver. The alloy can be MnGa, CoMnGa, or Co₂MnGa. The thickness ofthe first epitaxial layer 110 can be prepared according to need. Thethickness of the first epitaxial layer 110 can be in a range from about100 nanometers to about 500 micrometers. For example, the thickness ofthe first epitaxial layer 110 can be about 200 nanometers, 500nanometers, 1 micrometer, 2 micrometers, 5 micrometers, 10 micrometers,or 50 micrometers.

The step (S12) includes the following substeps:

(S121) placing the substrate 100 with the graphene layer 120 thereoninto a reaction chamber and heating the substrate 100 to 1100° C.˜1200°C., introducing the carrier gas and baking the substrate 100 for about200 seconds to about 1000 seconds;

(S122) cooling down the temperature to a range from about 500° C. to650° C. in the carrier gas atmosphere, introducing the Ga source gas andthe nitrogen source gas at the same time to grow the low-temperature GaNlayer;

(S123) stopping introducing the Ga source gas in the carrier gas andnitrogen source gas atmosphere, increasing the temperature to a rangefrom about 1100° C. to about 1200° C. and keeping for about 30 secondsto about 300 seconds;

(S 124) keeping the temperature of the substrate 100 in a range fromabout 1000° C. to about 1100° C., introducing the Ga source gas againand the Si source gas to grow the high quality first epitaixal layer110.

Furthermore, a step of growing an intrinsic epitaxial layer between step(S122) and step (S123). The intrinsic epitaxial layer can further reducethe dislocation during growing the first epitaxial layer 110.

In step (S13), the graphene layer 120 is directly in contact with thesubstrate 100. The graphene layer 120 can include at least one graphenefilm. The graphene film, namely a single-layer graphene, is a singlelayer of continuous carbon atoms. The single-layer graphene is ananometer-thick two-dimensional analog of fullerenes and carbonnanotubes. When the graphene layer 120 includes the at least onegraphene film, a plurality of graphene films can be stacked on eachother or arranged coplanar side by side. The graphene film can bepatterned by cutting or etching. The thickness of the graphene layer 120can be in a range from about 1 nanometer to about 100 micrometers. Forexample, the thickness of the graphene layer 120 can be 1 nanometer, 10nanometers, 200 nanometers, 1 micrometer, or 10 micrometers. Thesingle-layer graphene can have a thickness of a single carbon atom. Inone embodiment, the graphene layer 120 is a pure graphene structureconsisting of graphene.

The single-layer graphene has very unique properties. The single-layergraphene is almost completely transparent. The single-layer grapheneabsorbs only about 2.3% of visible light and allows most of the infraredlight to pass through. The thickness of the single-layer graphene isonly about 0.34 nanometers. A theoretical specific surface area of thesingle-layer grapheme is 2630 m²·g⁻¹. The tensile strength of thesingle-layer graphene is 125 GPa, and the Young's modulus of thesingle-layer graphene can be as high as 1.0 TPa. The thermalconductivity of the single-layer graphene is measured at 5300 W·m⁻¹·K⁻¹.A theoretical carrier mobility of the single-layer graphene is 2×10⁵cm²·V⁻¹·s⁻¹. A resistivity of the single-layer graphene is 1×10⁻⁶ Ω·cmwhich is about ⅔ of a resistivity of copper. Phenomenon of quantum Halleffects and scattering-free transmissions can be observed on thesingle-layer grapheme at room temperature.

In one embodiment, the graphene layer 120 is a patterned structure. Asshown in FIGS. 2-4, the term “patterned structure” means the graphenelayer 120 is a continuous structure and defines a plurality of apertures122. When the graphene layer 120 is located on the epitaxial growthsurface 101, part of the epitaxial growth surface 101 is exposed fromthe apertures 122 to grow the first epitaxial layer 110.

The shape of the aperture 122 is not limited and can be round, square,triangular, diamond or rectangular. The graphene layer 120 can have theapertures 122 of all the same shape or of different shapes. Theapertures 122 can be dispersed uniformly on the grapheme layer 102. Eachof the apertures 122 extends through the graphene layer 120 along thethickness direction. The apertures 122 can be circular shaped as shownin FIG. 2 or rectangular shaped as shown in FIG. 2. Alternatively, theapertures 122 can be a mixture of circular shaped and rectangular shapedin the patterned graphene layer 120, as shown in FIG. 4. Hereafter, thesize of the aperture 122 is the diameter of the hole or width of therectangular. The sizes of the apertures 122 can be different. Theaverage size of the apertures 122 can be in a range from about 10nanometers to about 500 micrometers. For example, the sizes of theapertures 122 can be about 50 nanometers, 100 nanometers, 500nanometers, 1 micrometer, 10 micrometers, 80 micrometers, or 120micrometers. The smaller the sizes of the apertures 122, the lessdislocation defects will occur during the process of growing the firstepitaxial layer 110. In one embodiment, the sizes of the apertures 122are in a range from about 10 nanometers to about 10 micrometers. Adutyfactor of the graphene layer 120 is an area ratio between thesheltered epitaxial growth surface 101 and the exposed epitaxial growthsurface 101. The dutyfactor of the graphene layer 120 can be in a rangefrom about 1:100 to about 100:1. For example, the dutyfactor of thegraphene layer 120 can be about 1:10, 1:2, 1:4, 4:1, 2:1, or 10:1. Inone embodiment, the dutyfactor of the graphene layer 120 is in a rangefrom about 1:4 to about 4:1.

As shown in FIG. 5, the term “patterned structure” can also be aplurality of patterned graphene layers spaced from each other. Theaperture 122 is defined between adjacent two of the patterned graphenelayers. When the graphene layer 120 is located on the epitaxial growthsurface 101, part of the epitaxial growth surface 101 is exposed fromthe aperture 122 to grow the first epitaxial layer 110. In oneembodiment, the graphene layer 120 includes a plurality of graphenestrips placed in parallel with each other and spaced from each other asshown in FIG. 5.

The graphene layer 120 can be located on the epitaxial growth surface101 by transfer printing a preformed graphene film. The graphene filmcan be made by chemical vapor deposition, exfoliating graphite,electrostatic deposition, pyrolysis of silicon carbide, epitaxial growthon silicon carbide, or epitaxial growth on metal substrates.

In one embodiment, the graphene layer 120 can be made by followingsteps: step (131), making a graphene suspension with graphene powderdispersed therein; step (132), forming a continuous graphene coating onthe epitaxial growth surface 101 of the substrate 100; and

step (133), creating patterns on the continuous graphene coating.

In step (131), the powder is dispersed in a solvent such as water,ethanol, N-methylpyrrolidone, tetrahydrofuran, or 2-nitrogendimethylacetamide. The graphene powder can be made by graphite oxidereduction, pyrolysis of sodium ethoxide, cutting carbon nanotube, carbondioxide reduction method, or sonicating graphite. The concentration ofthe suspension can be in a range from about 1 mg/ml to about 3 mg/ml.

In step (132), the suspension can be coated on the pitaxial growthsurface 101 of the substrate 100 by spinning coating. The rotating speedof spinning coating can be in a range from about 3000 r/min to about5000 r/min. The time for spinning coating can be in a range from about 1minute to about 2 minutes.

In step (133), the method of creating patterns on the continuousgraphene coating can be photocatalytic titanium oxide cutting, ion beametching, atomic force microscope etching, or the plasma etching.

In one embodiment, photocatalytic titanium oxide cutting is used topattern the continuous graphene coating. The method includes followingsteps:

step (1331), making a patterned metal titanium layer;

step (1332), heating and oxidizing the patterned metal titanium layer toobtain a patterned titanium dioxide layer;

step (1333), contacting the patterned titanium dioxide layer with thecontinuous graphene coating;

step (1334), irradiating the patterned titanium dioxide layer withultraviolet light; and

step (1335), removing the patterned titanium dioxide layer.

In step (1331), the method of creating patterns on the graphene film canbe photocatalytic titanium oxide cutting, ion beam etching, atomic forcemicroscope etching, or the plasma etching. In one embodiment, an anodicaluminum oxide mask is placed on the surface of the graphene film, andthen the graphene film is etched by plasma. The anodic aluminum oxidemask has a plurality of micropores arranged in an array. The part of thegraphene film corresponding to the micropores of the anodic aluminumoxide mask will be removed by the plasma etching, thereby obtaining agraphene layer 120 having a plurality of apertures.

In one embodiment, photocatalytic titanium oxide cutting is used topattern the continuous graphene coating. The method includes followingsteps:

(a) making a patterned metal titanium layer;

(b) heating and oxidizing the patterned metal titanium layer to obtain apatterned titanium dioxide layer;

(c) contacting the patterned titanium dioxide layer with the continuousgraphene coating;

(d) irradiating the patterned titanium dioxide layer with ultravioletlight; and

(e) removing the patterned titanium dioxide layer.

In step (a), the patterned metal titanium layer can be formed by vapordeposition through a mask or photolithography on a surface of a quartzsubstrate. The thickness of the quartz substrate can be in a range fromabout 300 micrometers to about 1000 micrometers. The thickness of themetal titanium layer can be in a range from about 3 nanometers to about10 nanometers. In one embodiment, the quartz substrate has a thicknessof 500 micrometers, and the metal titanium layer has a thickness of 4nanometers. The patterned metal titanium layer is a continuous titaniumlayer having a plurality of spaced stripe-shaped openings.

In step (b), the patterned metal titanium layer is heated underconditions of about 500° C. to about 600° C. for about 1 hour to about 2hours. The heating can be performed on a furnace.

In step (d), the ultraviolet light has a wavelength of about 200nanometers to about 500 nanometers. The patterned titanium dioxide layeris irradiated by the ultraviolet light in air or oxygen atmosphere withhumidity of about 40% to about 75%. The irradiating time is about 30minutes to about 90 minutes. Because the titanium dioxide is asemiconductor material with photocatalytic property, the titaniumdioxide can produce electrons and holes under ultraviolet lightirradiation. The electrons will be captured by the Ti (IV) of thetitanium surface, and the holes will be captured by the lattice oxygen.Thus, the titanium dioxide has strong oxidation-reduction ability. Thecaptured electrons and holes are easy to oxidize and reduce the watervapor in the air to produce active substance such as O₂ and H₂O₂. Theactive substance can decompose the graphene material easily.

In step (e), the patterned titanium dioxide layer can be removed byremoving the quartz substrate. After removing the patterned titaniumdioxide layer, the patterned graphene layer 120 can be obtained. Thepattern of the patterned graphene layer 120 and the pattern of thepatterned titanium dioxide layer are mutual engaged with each other.Namely, the part of the continuous graphene coating corresponding to thepatterned titanium dioxide layer will be removed off.

In other embodiment, in step (a), the patterned metal titanium layer canbe formed by depositing titanium on a patterned carbon nanotubestructure directly. The carbon nanotube structure can be a carbonnanotube film or a plurality of carbon nanotube wires. The plurality ofcarbon nanotube wires can be crossed or weaved together to form a carbonnanotube structure. The plurality of carbon nanotube wires can also belocated in parallel and spaced from each other to form a carbon nanotubestructure. Because a plurality of apertures is formed in the carbonnanotube film or between the carbon nanotube wires, the carbon nanotubestructure can be patterned. The titanium deposited on the patternedcarbon nanotube structure can form a patterned titanium layer. In step(b), the patterned titanium layer can be heated by applying an electriccurrent through the patterned carbon nanotube structure. In step (d),the part of the continuous graphene coating corresponding to thepatterned carbon nanotube structure will be removed off to form aplurality of apertures 122. Because the diameter of the carbon nanotubeis about 0.5 nanometers to about 50 nanometers, the size of theapertures 122 can be several nanometers to tens nanometers. The size ofthe apertures 122 can be controlled by selecting the diameter of thecarbon nanotube.

The carbon nanotube structure is a free-standing structure. The term“free-standing structure” means that the carbon nanotube structure cansustain the weight of itself when it is hoisted by a portion thereofwithout any significant damage to its structural integrity. That is, thecarbon nanotube structure can be suspended by two spaced supports. Thus,the process of creating patterns on the continuous graphene coating canbe operated as follow. For example, first depositing titanium layer on aplurality of parallel carbon nanotube wires; second heating andoxidizing the titanium layer on the plurality of carbon nanotube wiresform titanium dioxide layer; third locating the plurality of carbonnanotube wires on the continuous graphene coating; fourth irradiatingthe plurality of carbon nanotube wires with the ultraviolet light; lastremoving the plurality of carbon nanotube wires to obtain a graphenelayer 120 having a plurality of rectangular shaped apertures 122.

In one embodiment, the carbon nanotube structure includes at least onedrawn carbon nanotube film. A drawn carbon nanotube film can be drawnfrom a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 6-7, each drawn carbon nanotubefilm includes a plurality of successively oriented carbon nanotubesegments 143 joined end-to-end by van der Waals attractive forcetherebetween. Each carbon nanotube segment 143 includes a plurality ofcarbon nanotubes 145 parallel to each other, and combined by van derWaals attractive force therebetween. As can be seen in FIG. 8, somevariations can occur in the drawn carbon nanotube film. The carbonnanotubes 145 in the drawn carbon nanotube film are oriented along apreferred orientation. The drawn carbon nanotube film can be treatedwith an organic solvent to increase the mechanical strength andtoughness and reduce the coefficient of friction of the drawn carbonnanotube film. A thickness of the drawn carbon nanotube film can rangefrom about 0.5 nanometers to about 100 micrometers. The drawn carbonnanotube film can be attached to the epitaxial growth surface 101directly.

The carbon nanotube structure can include at least two stacked drawncarbon nanotube films. In other embodiments, the carbon nanotubestructure can include two or more coplanar carbon nanotube films, andcan include layers of coplanar carbon nanotube films. Additionally, whenthe carbon nanotubes in the carbon nanotube film are aligned along onepreferred orientation (e.g., the drawn carbon nanotube film), an anglecan exist between the orientation of carbon nanotubes in adjacent films,whether stacked or adjacent. Adjacent carbon nanotube films can becombined by only the van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in twoadjacent carbon nanotube films can range from about 0 degrees to about90 degrees. When the angle between the aligned directions of the carbonnanotubes in adjacent stacked drawn carbon nanotube films is larger than0 degrees, a plurality of micropores is defined by the carbon nanotubestructure. Referring to FIG. 10, the carbon nanotube structure is shownwith the aligned directions of the carbon nanotubes between adjacentstacked drawn carbon nanotube films at 90 degrees. Stacking the carbonnanotube films will also add to the structural integrity of the carbonnanotube structure.

A step of heating the drawn carbon nanotube film can be performed todecrease the thickness of the drawn carbon nanotube film. The drawncarbon nanotube film can be partially heated by a laser or microwave.The thickness of the drawn carbon nanotube film can be reduced becausesome of the carbon nanotubes will be oxidized. In one embodiment, thedrawn carbon nanotube film is irradiated by a laser device in anatmosphere comprising of oxygen therein. The power density of the laseris greater than 0.1×10⁴ watts per square meter. The drawn carbonnanotube film can be heated by fixing the drawn carbon nanotube film andmoving the laser device at a substantially uniform speed to irradiatethe drawn carbon nanotube film. When the laser irradiates the drawncarbon nanotube film, the laser is focused on the surface of the drawncarbon nanotube film to form a laser spot. The diameter of the laserspot ranges from about 1 micron to about 5 millimeters. In oneembodiment, the laser device is carbon dioxide laser device. The powerof the laser device is about 30 watts. The wavelength of the laser isabout 10.6 micrometers. The diameter of the laser spot is about 3millimeters. The velocity of the laser movement is less than 10millimeters per second. The power density of the laser is 0.053×10¹²watts per square meter.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into an untwisted carbon nanotube wire. Theuntwisted carbon nanotube wire includes a plurality of carbon nanotubessubstantially oriented along a same direction (i.e., a direction alongthe length of the untwisted carbon nanotube wire). The carbon nanotubesare substantially parallel to the axis of the untwisted carbon nanotubewire. More specifically, the untwisted carbon nanotube wire includes aplurality of successive carbon nanotube segments joined end to end byvan der Waals attractive force therebetween. Each carbon nanotubesegment includes a plurality of carbon nanotubes substantially parallelto each other, and combined by van der Waals attractive forcetherebetween. The carbon nanotube segments can vary in width, thickness,uniformity, and shape. The length of the untwisted carbon nanotube wirecan be arbitrarily set as desired. A diameter of the untwisted carbonnanotube wire ranges from about 0.5 nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. The twistedcarbon nanotube wire includes a plurality of carbon nanotubes helicallyoriented around an axial direction of the twisted carbon nanotube wire.More specifically, the twisted carbon nanotube wire includes a pluralityof successive carbon nanotube segments joined end to end by van derWaals attractive force therebetween. Each carbon nanotube segmentincludes a plurality of carbon nanotubes parallel to each other, andcombined by van der Waals attractive force therebetween. The length ofthe carbon nanotube wire can be set as desired. A diameter of thetwisted carbon nanotube wire can be from about 0.5 nanometers to about100 micrometers. Further, the twisted carbon nanotube wire can betreated with a volatile organic solvent after being twisted to bundlethe adjacent paralleled carbon nanotubes together. The specific surfacearea of the twisted carbon nanotube wire will decrease, while thedensity and strength of the twisted carbon nanotube wire will increase.

The graphene layer 120 can also be composed of graphene and additives.The additives can be carbon nanotube, Si₃N₄, BN, SiC, or SiO₂. Theadditives can be added into the graphene via chemical vapor deposition,physical vapor deposition, or magnetron sputtering. Furthermore, theepitaxial growth surface 101 can also be divided into a wetting area andnon-wetting area, and the graphene is directly coated on the epitaxialgrowth surface 101 to form the graphene layer 120.

The graphene layer 120 can be used as a mask for growing the secondepitaxial layer 130. The mask is the patterned graphene layer 120sheltering a part of the first epitaxial layer 110 and exposing anotherpart of the first epitaxial layer 110. Thus, the second epitaxial layer130 can grow from the exposed first epitaxial layer 110 The graphenelayer 120 can form a patterned mask on the first epitaxial layer 110because the patterned graphene layer 120 defines a plurality ofapertures 122. Compared to lithography or etching, the method of forminga patterned graphene layer 120 as mask is simple, low in cost, and willnot pollute the substrate 100.

In step (S14), the method for growing the second epitaxial layer 130 canbe same as the method of growing the first epitaxial layer 110. Thematerial of the second epitaxial layer 130 can be same as the materialof the first epitaxial layer 110, and the second epitaxial layer 130 andthe first epitaxial layer 110 can be integrated to form an integratedstructure, and the graphene layer 120 is embedded into the integratedstructure. The second epitaxial layer 130 can be grown by a method suchas molecular beam epitaxy, chemical beam epitaxy, reduced pressureepitaxy, low temperature epitaxy, select epitaxy, liquid phasedeposition epitaxy, metal organic vapor phase epitaxy, ultra-high vacuumchemical vapor deposition, hydride vapor phase epitaxy, or metal organicchemical vapor deposition (MOCVD). The thickness of second epitaxiallayer 130 can range from about 0.5 nanometers to about 1 millimeter,such as 100 nanometers, 200 nanometers to about 200 micrometers, 500nanometers, 100 micrometers, 200 micrometers, or 500 micrometers.

The growth of the second epitaxial layer 130 includes following stages:

First stage, a plurality of epitaxial crystal nucleus forms on theentire exposed surface of the first epitaxial layer 110, and theepitaxial crystal nucleus grows to a plurality of epitaxial crystalgrains 1302 along the direction perpendicular the first epitaxial layer110;

Second stage, the plurality of epitaxial crystal grains 1302 grows to acontinuous epitaxial film 1304 along the direction parallel to the firstepitaxial layer 110;

Third stage, the epitaxial film 1304 continuously grows along thedirection perpendicular to the first epitaxial layer 110 to form thesecond epitaxial layer 130.

In the first stage, because the graphene layer 120 is located on thesecond epitaxial growth surface 1011 and defines a plurality ofapertures 122, the epitaxial crystal grains 1302 are grown from theexposed first epitaxial layer 110 through the apertures 122. The processof epitaxial crystal grains 1302 which grow along the directionsubstantially perpendicular to the first epitaxial layer 110 is calledvertical epitaxial growth. The epitaxial crystal grains 1302 grow fromthe apertures 122 of the graphene layer 120.

In the second stage, the epitaxial crystal grains 1302 can grow alongthe direction parallel to the first epitaxial layer 110. The epitaxialcrystal grains 1302 are gradually joined together to form the epitaxialfilm 1304 to cover the graphene layer 120. During the growth process,the epitaxial crystal grains 1302 will grow around the graphene, andthen a plurality of holes will be formed in the second epitaxial layer130 where the graphene existed.

In the third stage, the second epitaxial layer 130 covers the graphenelayer 120, and contacts with the first epitaxial layer 110 through theapertures 122. Due to the graphene layer 120, the lattice dislocationbetween the epitaxial crystal grains and the first epitaxial layer 110will be reduced during the growing process, thus the second epitaxiallayer 130 has less defects therein.

Furthermore, a step of applying another graphene layer (not shown) onthe epitaxial growth surface 101 before growing the first epitaxiallayer 110 can be performed. Thus the quality of the first epitaxiallayer 110 and the second epitaxial layer 130 can be improved.

Referring to FIGS. 10 and 11, an epitaxial structure 10 provided in oneembodiment includes a substrate 100 having an epitaxial growth surface101, a first epitaxial layer 110, a graphene layer 120, and secondepitaxial layer 130 stacked on the epitaxial growth surface 101 in thatorder. The graphene layer 120 is located on the epitaxial growth surface101 and defines a plurality of apertures 122. A first part of the firstepitaxial layer 110 is exposed through the plurality of apertures 122,and a second part of the first epitaxial layer 110 is covered by thegraphene layer 120. The second epitaxial layer 130 is located on thegraphene layer 120 and contacts the exposed surface of the firstepitaxial layer 110 through the apertures 122.

The second epitaxial layer 130 defines a plurality of holes adjacent toand oriented to the first epitaxial layer 110. The holes can be blindholes or grooves. The holes and the first epitaxial layer 110cooperatively form a sealed chamber to receive the graphene layer 120therein. The inner wall of the holes can be in contact with the graphenelayer 120. In one embodiment, the graphene layer 120 includes aplurality of graphene strips located in parallel with each other andspaced from each other.

Referring to FIGS. 12 and 13, an epitaxial structure 20 provided in oneembodiment includes a substrate 100, a first epitaxial layer 110, agraphene layer 120, and second epitaxial layer 130 stacked on thesubstrate 100 in that order. The epitaxial structure 20 is similar tothe epitaxial structure 10 above except that the graphene layer 120 is agraphene film having a plurality of circular shaped apertures 122arranged in an array. Part of the second epitaxial layer 130 extendsthrough the hole-shaped apertures 122 and in contact with the firstepitaxial layer 110.

A method for making the epitaxial structure 20 of one embodiment issimilar with the method for making the epitaxial structure 10, exceptthat the graphene layer 120 is made of single graphene film. The methodof making graphene layer 120 includes following steps:

(S141) providing a graphene film;

(S142) transferring the graphene film on the patterned epitaixal growthsurface of the substrate 100; and

(S143) creating patterns on the graphene film.

In step (141), the graphene film is made by chemical vapor depositionwhich includes the steps of: (a1) providing a substrate; (b1) depositinga metal catalyst layer on the substrate; (c1) annealing the metalcatalyst layer; and (d1) growing the graphene film in a carbon sourcegas.

In step (a1), the substrate can be a copper foil or a Si/SiO₂ wafer. TheSi/SiO₂ wafer can have a Si layer with a thickness in a range from about300 micrometers to about 1000 micrometers and a SiO₂ layer with athickness in a range from about 100 nanometers to about 500 nanometers.In one embodiment, the Si/SiO₂ wafer has a Si layer with a thickness ofabout 600 micrometers and a SiO₂ layer with a thickness of about 300nanometers.

In step (b1), the metal catalyst layer can be made of nickel, iron, orgold. The thickness of the metal catalyst layer can be in a range fromabout 100 nanometers to about 800 nanometers. The metal catalyst layercan be made by chemical vapor deposition, physical vapor deposition,such as magnetron sputtering or electron beam deposition. In oneembodiment, a metal nickel layer of about 500 nanometers is deposited onthe SiO₂ layer.

In step (c1), the annealing temperature can be in a range from about900° C. to about 1000° C. The annealing can be performed in a mixed gasof argon gas and hydrogen gas. The flow rate of the argon gas is about600 sccm, and the flow rate of the hydrogen gas is about 500 sccm. Theannealing time is in a range from about 10 minutes to about 20 minutes.

In step (d1), the growth temperature is in a range from about 900° C. toabout 1000° C. The carbon source gas is methane. The growth time is in arange from about 5 minutes to about 10 minutes.

In step (142), the transferring the graphene film includes the steps of:(a2) coating an organic colloid or polymer on the surface of thegraphene film as a supporter; (b2) baking the organic colloid or polymeron the graphene film; (c2) immersing the baked graphene film with theSi/SiO₂ substrate in deionized water so that the metal catalyst layerand the SiO₂ layer was separated to obtain a supporter/graphenefilm/metal catalyst layer composite; (d2) removing the metal catalystlayer from the supporter/graphene film/metal catalyst layer composite toobtain a supporter/graphene film composite; (e2) placing thesupporter/graphene film composite on the epitaxial growth surface 101;(f2) fixing the graphene film on the epitaxial growth surface 101 firmlyby heating; and (g2) removing the supporter.

In step (a2), the supporter material is poly (methyl methacrylate)(PMMA), polydimethylsiloxane, positive photoresist 9912, or photoresistAZ5206.

In step (b2), the baking temperature is in a range from about 100° C. toabout 185° C.

In step (c2), an ultrasonic treatment on the metal catalyst layer andthe SiO₂ layer can be performed after being immersed in deionized water.

In step (d2), the metal catalyst layer is removed by chemical liquidcorrosion. The chemical liquid can be nitric acid, hydrochloric acid,ferric chloride (FeCl₃), and ferric nitrate (Fe(NO₃)₃).

In step (g2), the supporter is removed by soaking the supporter inacetone and ethanol first, and then heating the supporter to about 400°C. in a protective gas.

In step (143), the method of creating patterns on the graphene film canbe photocatalytic titanium oxide cutting, ion beam etching, atomic forcemicroscope etching, or the plasma etching. In one embodiment, an anodicaluminum oxide mask is placed on the surface of the graphene film, andthen the graphene film is etched by plasma. The anodic aluminum oxidemask has a plurality of micropores arranged in an array. The part of thegraphene film corresponding to the micropores of the anodic aluminumoxide mask will be removed by the plasma etching, thereby obtaining agraphene layer 120 having a plurality of apertures 122.

Furthermore, another graphene layer (not shown) can be sandwichedbetween the substrate 100 and the first epitaxial layer 110 to improvethe quality of the first epitaxial layer 110.

Referring to FIG. 14, an epitaxial structure 30 provided in oneembodiment includes a first epitaxial layer 110, a graphene layer 120,and a second epitaxial layer 130 stacked on a substrate 100 in thatorder. The epitaxial structure 30 is similar to the epitaxial structure10 above except that the graphene layer 120 is graphene powder dispersedon the epitaxial growth surface of the substrate 100. The graphenepowder can be grown on the substrate 100 directly.

The epitaxial structure and method for growing the epitaxial layer onthe epitaxial structure has following advantages. First, the substrateis a patterned structure having a plurality of grooves in micrometerscale, so the dislocation during the growth will be reduced. Second, thegraphene layer is a patterned structure, the thickness and the apertureis in nanometer scale, thus the dislocation is further reduced and thequality of the epitaxial layer is improved. Third, due to the existenceof the graphene layer, the contact surface between the epitaxial layerand the substrate will be reduced, and the stress between them isreduced, thus the substrate can be used to grow a relatively thickerepitaxial layer. Fourth, the graphene layer is a freestanding structure,thus it can be directly placed on the substrate, and the method issimple and low in cost. Fifth, the epitaxial layer grown on thesubstrate has relatively less dislocation, so it can be used to produceelectronics in higher performance.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. An epitaxial structure, comprising: a substratehaving an epitaxial growth surface; a first epitaxial layer on theepitaxial growth surface; a graphene layer on the first epitaxial layer;and a second epitaxial layer on the first epitaxial layer and coveringthe graphene layer.
 2. The epitaxial structure of claim 1, wherein thegraphene layer is a graphene structure consisting of graphene.
 3. Theepitaxial structure of claim 2, wherein the graphene structure comprisesa graphene film consisting of a single layer of continuous carbon atoms.4. The epitaxial structure of claim 3, wherein the graphene layer has athickness same as a single carbon atom.
 5. The epitaxial structure ofclaim 1, wherein a thickness of the graphene layer is in a range fromabout 1 nanometer to about 100 micrometers.
 6. The epitaxial structureof claim 1, wherein the graphene layer defines a plurality of aperturesto expose a part of a surface of the first epitaxial layer, and thesecond epitaxial layer is located on the graphene layer and contacts thefirst epitaxial layer through the plurality of apertures.
 7. Theepitaxial structure of claim 6, wherein sizes of the plurality ofapertures are in a range from about 10 nanometers to about 10micrometers.
 8. The epitaxial structure of claim 6, wherein a dutyfactorof the graphene layer is in a range from about 1:100 to about 100:1,wherein the dutyfactor is an area ratio between a covered part to theexposed part of the epitaxial growth surface.
 9. The epitaxial structureof claim 1, wherein a material of the first epitaxial layer is same as amaterial of the second epitaxial layer, and the first epitaxial layerand the second epitaxial layer are an integrated structure.
 10. Theepitaxial structure of claim 9, wherein the graphene layer is embeddedinto the integrated structure.
 11. The epitaxial structure of claim 1,wherein the epitaxial structure comprises a second graphene layersandwiched between the substrate and the first epitaxial layer.
 12. Theepitaxial structure of claim 1, wherein the graphene layer comprises aplurality of graphene strips spaced from each other, and an aperture isdefined between each adjacent two of the plurality of graphene strips.13. The epitaxial structure of claim 1, wherein the graphene layer is acoating comprises graphene powders.
 14. The epitaxial structure of claim1, wherein a material of the first epitaxial layer is semiconductor,metal or alloy.
 15. The epitaxial structure of claim 1, wherein amaterial of the substrate is GaAs, GaN, AlN, Si, SOI, SiC, MgO, ZnO,LiGaO₂, LiAlO₂, or Al₂O₃.
 16. The epitaxial structure of claim 1,wherein the second epitaxial layer defines a plurality of holes, and thegraphene layer is received into the plurality of holes.
 17. A epitaxialstructure, comprising: a substrate having an epitaxial growth surface; afirst graphene layer, a patterned graphene layer, and a second epitaxiallayer sequentially stacked on the epitaxial growth surface; wherein thepatterned graphene layer is sandwiched between the first epitaxial layerand the second epitaxial layer, and the patterned graphene layer definesa plurality apertures passing through the patterned graphene layer alonga direction of the thickness of the patterned graphene layer.